The present invention relates to a tomographic imaging method and apparatus for a human body, which utilize the phenomenon of nuclear magnetic resonance and which are used for medical diagnoses.
Prior-art techniques close to the present invention are broadly classified into the following two:
[1] Half-Fourier method PA1 [2] Symmetric conjugated method PA1 (i) measuring a signal in the upper half surface (with respect to a phase encode direction) of a region which ought to be measured. PA1 (ii) further measuring several lines below phase encoding O-line so as to determine the value of a phase information parameter Phi, PA1 (iii) putting a value 0 (zero) in the remaining unmeasured region (the lower half surface), PA1 (iv) Fourier-transforming the values of the measured region constructed by the steps (i) and (iii), and PA1 (v) multiplying an obtained image by a term dependent upon the parameter Phi, the real part of the resultant image being regarded as a reconstructed image. PA1 (vi) measuring signals in the upper half surface of a measurement region and several lines below phase encoding O-line, PA1 (vii) giving each point of the remaining unmeasured region the conjugate value of the signal of a measurement point symmetric to that point with respect to an origin, as the estimated value of a signal which ought to have been measured, and PA1 (viii) Fourier-transforming the values of the measured region constructed by the steps (vi) and (vii), a resultant image being regarded as a reconstructed image. PA1 (a) As indicated in the steps (i) and (ii), the region to measure the signals must be limited to the region which consists of the upper (or lower) surface and several central lines. PA1 (b) In the steps (ii) and (v), the phase correction is made using the single parameter Phi. With this process, it is not considered that, in general, phases change complicatedly at various points of an image. As a result, the process forms the cause of an inferior image quality. PA1 (c) In the estimation of the step (vii), no phasic effect is taken into consideration. As a result, this process forms the cause of an inferior image quality. PA1 (i) An RF pulse 1001 for magnetic resonance is impressed simultaneously with the application of a gradient magnetic field 1002 in a z-direction, thereby to excite the magnetization of a specified slice plane. Further, a gradient magnetic field 1003 in the z-direction is applied, thereby to make the phases of the magnetization uniform. PA1 (ii) Thereafter, a gradient magnetic field 1004 in a y-direction is applied for phase encoding. PA1 (iii) A gradient magnetic field 1005 in an x-direction is simultaneously applied, followed by the application of an x-directional gradient magnetic field 1006, during which an MR signal 1007 is measured. PA1 (iv) The gathered MR signals 1007 are subjected to the Fourier transformation, thereby to obtain the reconstructed image. PA1 (i) An RF pulse 1101 for magnetic resonance is impressed simultaneously with the application of a z-directional gradient magnetic field 1102, thereby to excite the magnetization of the specified slice plane. Further, a z-directional gradient magnetic field 1103 is applied, thereby to make the phases of the magnetization uniform. PA1 (ii) Thereafter, a y-directional gradient magnetic field 1104 is applied for phase encoding. PA1 (iii) The application of a series of x-directional gradient magnetic fields 1105, 1106 and 1107 is simultaneously started, and an M signal 1108 is measured during the application of the x-directional gradient magnetic field 1107. PA1 (iv) The gathered MR signals 1108 are subjected to the Fourier transformation, thereby to obtain the reconstructed image. PA1 (1) In an actual blood vessel, especially the artery, the blood stream velocity is always changing due to pulsation. PA1 (2) Even if the blood stream speed is substantially constant, the blood vessel is moving while depicting a complicated curve, and hence, the blood stream velocity as a vector including the direction is changing with time. That is, the differential of higher order of the blood stream velocity with respect to time is not necessarily zero. PA1 (i) For a measurement region E required by the prior-art method, any desired partial region E+ is set (here, the combination between the region E+ and a region obtained by moving the region E+ in point symmetry with respect to the origin of a measurement space includes the region E). PA1 (ii) A signal h.sup.+ is measured on the region E+. PA1 (iii) An image is reconstructed from the signals h.sup.+ on a region neighboring the origin, thereby to obtain a phase image .theta.. PA1 (iv) An image obtained by computing the following expression (1) is regarded as a reconstructed image: ##EQU1## where F denotes the operation of Fourier transformation, Q.sub.1 the operation of multiplication by e.sup.2i.phi., Q.sub.1 the operation of multiplication by e.sup.-2i.phi., Q.sub.2 the operation of F E.sup.- F.sup.-1, Q.sub.2 the operation of F.sup.-1 E.sup.- F, E.sup.- the operation of multiplication by 0 on the region E.sup.+ and by 1 on the other region, and the operation of synthesizing operations. In addition, `-` indicates complex conjugate, `n` an integer of at leas 0 expressive of the number of times of repetition of operations, and `N` an integer of at least 0 expressive of the total number of the terms of a series. PA1 1. A measured signal is obtained according to a sequence shown in (a), (b), (c), (d) and (e) of FIG. 8. PA1 2. The MR signals 807 gathered are subjected to the MR half-Fourier reconstruction method stated before (however, the phase encode direction y and the readout direction x in the prior-art method are handled conversely), whereby a reconstructed image is obtained. Hereinafter, this image shall be called the "short insensitive image." PA1 3. A measured signal is obtained according to a sequence shown in (a), (b), (c), (d') and (e) of FIG. 8. PA1 4. The MR signals 807 gathered are subjected to the MR half-Fourier reconstruction method (however, the phase encode direction y and the readout direction in the prior-art method are handled conversely), whereby a reconstructed image is obtained. Hereinafter, this image shall be called the "short sensitive image." PA1 5. The short sensitive image obtained in the aforementioned operation 4 is subtracted from the short insensitive image obtained in the aforementioned operation 2. Then, the image of blood vessels is obtained. PA1 6. A measured signal is obtained according to a sequence shown in (a), (b), (c), (d) and (e) of FIG. 9. PA1 7. The MR signals 908 gathered are subjected to the MR half-Fourier reconstruction method (however, the phase encode direction y and the readout direction x in the prior-art method are handled conversely), thereby to obtain a reconstructed image. Hereinafter, this image shall be called the "second short insensitive image." PA1 8. A measured signal is obtained according to a sequence shown in (a), (b), (c), (d') and (e) of FIG. 9. PA1 9. The MR signals 908 gathered are subjected to the MR half-Fourier reconstruction method (however, the phase encode direction y and the readout direction x in the prior-art method are handled conversely), thereby to obtain a reconstructed image. Hereinafter this image shall be called the "second short sensitive image." PA1 10. The second short sensitive image obtained in the aforementioned operation 9 is subtracted from the second short insensitive image obtained in the aforementioned operation 7. Then, the image of blood vessels is obtained. PA1 11. The same operation as the above-stated operation 1 (or operation 6) is carried out. However, the magnetic resonance RF pulse 801 (or 901) in (a) of FIG. 8 (or (a) of FIG. 9) is specified to an RF pulse for throwing down the magnetization 360.degree.. PA1 12. The same operation as the above-stated operation 2 (or operation 7) is carried out. A reconstructed image thus obtained is the image of blood vessels.
The method [1] is stated in "SPIE," Vol. 593 (1985), pp. 6-13.
The method [2] is stated in "Radiology," May (1986), pp. 527-531.
The half-Fourier method [1] consists roughly of the steps of:
The symmetric conjugated method [2] consists of the steps of:
The half-Fourier method [1] has had the following problems:
The symmetric conjugated method [2] has had the following problem in addition to the above problem (a):
Further, prior-art technology for imaging blood vessels is stated in "Trans. Med. Imaging," vol. MI-5, No. 3, pp. 140-151, 1986. As referred to before, the prior art concerning the image reconstruction method by which an image fundamentally the same as a reconstructed image based on the Fourier transformation method is obtained from data items in a number equal to about a half of the number of data items necessary for the Fourier transformation method and the phase information of the image (hereinbelow, this reconstruction method shall be termed the "MR half-Fourier reconstruction method") is discussed in "SPIE," Vol. 593 (1985), Medical Image Processing, pp. 6-13.
Regarding the prior-art blood vessel imaging technology, the technique closest to the present invention among techniques discussed in the aforementioned thesis will now be explained.
First, an image called "sensitive image" is obtained according to a sequence shown in FIG. 10. The steps of the process are as follows:
Subsequently, an image called "insensitive image" is obtained according to a sequence shown in FIG. 11. The steps of the process are as follows:
Next, the sensitive image is subtracted from the insensitive image. The image of the blood vessels is obtained by the above operations, the principle of which is as explained below.
Referring to FIG. 10, the phase rotation of the magnetization ascribable to the application of the x-directional gradient field 1005 and the application of the x-directional gradient field 1006 for a preceding time interval T does not occur in a stationary part because the amounts of application of the former and latter magnetic fields cancel each other. In a blood stream part, however, the magnetization moves inside the gradient fields with velocities, and the amounts of application of the former and latter magnetic fields are not perfectly canceled, so that the phase rotation .phi. of the magnetization does not become zero but takes various values, depending upon a blood stream velocity v. Assuming that the velocity v be constant irrespective of time and be in the x-direction, the phase rotation .phi. becomes: EQU .phi.=6 .gamma.T.sup.2 G.sub.x .vertline.v.vertline.
where .gamma. denotes a constant, G.sub.x the gradient of the x-directional gradient field, and T the duration of the application of the x-directional gradient field 1005. Since, however, the blood stream velocity is not constant but exhibits a certain distribution in the direction of the thickness of a slice, the signal actually measured becomes the sum of signals whose phases are distributed at random in the thickness direction of the slice and which cancel one another to afford a small value. In the sensitive image, accordingly, the blood vessels come out dark.
On the other hand, referring to FIG. 11, the phase rotation of the magnetization ascribable to the application of the series of x-directional gradient fields 1105, 1106 and 1107 does not occur in the stationary part. When the blood stream velocity v is assumed to be constant irrespective of time and to be in the x-direction, the phase rotation of the magnetization does not occur in the blood stream part, either. The reason is as stated below. The phase rotation .phi.' ascribable to the application of the x-directional gradient field 1005 and the application of the x-directional gradient field 1006 for a preceding time interval T becomes: EQU .phi.'=-2 .gamma.T.sup.2 G.sub.x .vertline.v.vertline.
on the same ground as in the case of FIG. 10. Likewise, the phase rotation .phi." ascribable to the application of the x-directional gradient field 1006 for a succeeding time interval T and the application of the x-directional gradient field 1007 for a preceding time interval T becomes: EQU .phi."=+2 .gamma.T.sup.2 G.sub.x .vertline.v.vertline.
Eventually, the overall phase rotation .phi. becomes: EQU .phi.=.phi.'+.phi."=0
Accordingly, the phases do not become random depending upon the stream velocity, so that the blood vessels come out bright in the insensitive image.
Therefore, when the sensitive image is subtracted from the insensitive image, the images of the stationary part are canceled, and only the blood vessels are imaged.
As regards the prior art for imaging blood vessels, another technique will now be described. This technique is characterized in that the angle of the magnetization of a stationary part to be thrown down by an RF pulse for magnetic resonance is set at 360.degree.. Thus, the magnetization of the stationary part is not excited, whereas the magnetization of a blood stream part is excited because it is not perfectly thrown down 360.degree. on account of the stream thereof. Accordingly, when echoes are thereafter caused so as to measure signals, only the signals from the blood stream part are obtained. By way of example, concrete steps may be quite the same as the foregoing steps for obtaining the insensitive image in accordance with the sequence shown in FIG. 11, except that the magnetic resonance RF pulse 1101 in (a) of FIG. 11 is specified to the RF pulse for throwing down the magnetization 360.degree.. Owing to these steps, the blood vessels are imaged by the signal insensitive image.
The prior art for imaging blood vessels is as thus far explained. Now, there will be explained a prior-art technique concerning the MR Half Scan reconstruction method.
All echo signals to-be-measured which are required for attaining a desired resolution when the image reconstructing method is the two-dimensional Fourier transformation method, shall be called the "whole echo signal." On this occasion, when the two-dimensional Fourier transformation method is improved using information on the phase of a reconstructed image, for example, that the reconstructed image is of a real number, substantially the same image as a reconstructed image which is obtained by the two-dimensional Fourier transformation of the whole echo signal can be reconstructed out of data items the number of which is equal to about a half of the number of the data items of the whole echo signal. When this fact is utilized, imaging can be carried out by reducing the number of times of phase encode steps to about a half and without incurring any appreciable degradation in the image quality.
In the prior art for imaging blood vessels, points (1) and (2) mentioned below are not taken into consideration because of the assumption that the blood stream velocity v is constant versus time. Accordingly, the prior art has had the problem that the value of the phase rotation .phi. of the blood stream part in the sequence shown in FIG. 11 is usually large for an actual complicated blood stream, so the blood vessels do not come out considerably bright even in the insensitive image.
On the other hand, the prior art concerning the MR Half Scan reconstruction method has been merely utilized for reducing the number of times of phase encode steps. Therefore, even when this technique is applied to the prior art for imaging blood vessels, the phase rotation of the magnetization in the sequence of each of FIGS. 10 and 11 does not differ at all from the phase rotation which arises without the application of the technique. Accordingly, the aforementioned problem of the prior art for imaging blood vessels remains intact.